A. Principles of Operation
Secondary cells using rechargeable hydrogen storage negative electrodes are an environmentally non-threatening, high energy density, electrochemical power source. Such hydrogen storage cells operate in a different manner than lead acid, nickel-cadmium, or other battery systems.
A rechargeable hydrogen storage electrochemical cell or battery utilizes a negative electrode that is capable of the reversible electrochemical storage of hydrogen. Hydrogen storage cells usually employ a positive electrode of nickel hydroxide material, although other positive materials may be used. The negative and positive electrodes are spaced apart in an alkaline electrolyte. A suitable separator, i.e., a membrane may also be positioned between the electrodes.
Upon application of an electrical potential across a hydrogen electrochemical cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxyl ion: EQU M+H.sub.2 O+e.sup.- .fwdarw.M-H+OH.sup.-.
Upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron: EQU M-H+OH.sup.- .fwdarw.M+H.sub.2 O+e.sup.-.
In the reversible (secondary) cells of the invention, the reactions are reversible.
The reactions that take place at the positive electrode of a secondary cell are also reversible. For example, the reactions at a conventional nickel hydroxide positive electrode as utilized in a hydrogen rechargeable secondary cell are: EQU Ni(OH).sub.2 +OH.sup.- .fwdarw.NiOOH+H.sub.2 +e.sup.- (Charging), EQU NiOOH+H.sub.2 O+e.sup.- .fwdarw.Ni(OH).sub.2 +OH.sup.- (Discharging).
A secondary cell utilizing an electrochemically rechargeable hydrogen storage negative electrode offers important advantages over conventional secondary cells and batteries, such as nickel-cadmium cells, lead-acid cells, and lithium cells. First, hydrogen storage secondary cells contain no cadmium, lead, or lithium; and thus, are not a hazard to consumers or to the environment. Second, electrochemical cells with hydrogen storage negative electrodes offer significantly higher specific charge capacities than do cells with lead or cadmium negative electrodes. As a result, a higher energy density is possible with hydrogen storage cells than with conventional systems, making hydrogen storage cells particularly suitable for many commercial applications.
B. AB.sub.2 Type Hydrogen Storage Alloys
A variety of hydrogen storage alloys, both electrochemical and thermal, are known in the art. One type of hydrogen storage alloy is the AB.sub.2 hydrogen storage alloys. Prior references teach basic C.sub.14 and C.sub.15 type Laves phase AB.sub.2 materials with (1) one or more of the following elements: Ti, Zr, and Hf; and (2) Ni, generally with one or more additional metals. However, there is no teaching in the prior art of the local metallurgical, chemical, or electrochemical relationships between the various individual metals that can partially substitute for Ti, Zr, and/or Hf; or for Ni. Nor is there any teaching of local, i.e., intra-phase, compositions or the effect of local compositional differences on catalytic properties and key determinants of catalytic properties, such as electron work function.
The earliest AB.sub.2 hydrogen storage materials were thermal hydrogen storage alloys. In thermal hydrogen storage alloys, the driving forces for hydriding and dehydriding are thermal and pressure driving forces. In contrast, electrochemical hydrogen storage alloys are hydrided and dehydrided by electron transfer processes in ionic media.
A. Pebler and E. A. Gulbransen, 239 Transactions of the Metallurgical Society, 1593-1600 (1967), first reported members of the AB.sub.2 class of materials to be the binaries ZrCr.sub.2, ZrV.sub.2, and ZrMo.sub.2. In addition, J. J. Reilly and R. H. Wiswall, in "The Reaction of Hydrogen with Alloys of Magnesium and Nickel and the Formation of Mg.sub.2 NiH.sub.4,"7 Inorganic Chem. 2254 (1968), reported that Mg-Ni thermal hydrogen storage alloys were also AB.sub.2 thermal hydrogen storage alloys that hydrided and dehydrided by pressure and temperature driven processes and not be electron transfer with an external circuit.
F. H. M. Spit, J. W. Drivjer, and S. Radelar described a ZrNi class of binary thermal hydrogen storage AB.sub.2 alloys in "Hydrogen Sorption by the Metallic Glass Ni.sub.64 Zr.sub.36 and by Related Crystalline Compounds," 14 Scripta Metallurgica 1071-1076 (1980); and the thermodynamics of gas phase hydrogen absorption and desorption in the ZrNi.sub.2 binary system. Subsequently, Spit, et al. in "Hydrogen Sorption in Amorphous Ni (Zr, Ti) Alloys," Zeitschrift Fur Physikaisch Chemie Neue Folge Bd. 225-232 (1979), reported the gas phase hydrogen sorption and desorption kinetics of thermal hydrogen storage processes in Zr.sub.36.3 Ni.sub.63.7 and Ti.sub.29 Zr.sub.9 Ni.sub.62.
Ziconium-manganese binary AB.sub.2 thermal hydrogen storage alloys were disclosed, for example, in F. Pourarian, H. Fuji, W. E. Wallace, V. K. Shina, and H. Kevin Smith, "Stability and Magnetism of Hydrides of Nonstoichiometric ZrMn.sub.2," 85 J. Phys. Chem 3105-3111. Pourarian, et al. describe a class of nonstoichiometric hydrides of the general formula ZrMn.sub.2+x where x=0.6, 0.8, and 1.8. (ZrTi)-manganese ternary hydrogen storage alloys were described by H. Fuji, F. Pourarian, V. K. Shina, and W. E. Wallace in "Magnetic, Crystallographic, and Hydrogen Storage Characteristics of Zr.sub.1-x Ti.sub.x Mn.sub.2 Hydrides," 85 J. Phys. Chem 3112.
Manganese-nickel binary AB.sub.2 thermal hydrogen storage alloys were described for thermal hydrogen storage in automotive applications by H. Buchner in "Perspectives for Metal Hydride Technology," 6 Prog. Energy Combust. Sci 331-346.
Ternary zirconium, nickel, manganese AB.sub.2 thermal hydrogen storage alloys were described, for example, by A. Suzuki and N. Nishimiya, "Thermodynamic Properties of Zr(Ni.sub.x Mn.sub.1-x)2-H.sub.2 Systems," 19 Mat. Res. Bull. 1559-1571 (1984). Suzuki, et al. describe the system Zr(Ni.sub.x Mn.sub.1-x).sub.2 where x=0.2, 0.5, and 0.8.
Six component AB.sub.2 thermal hydrogen storage alloys are described in German Patentschrift DE 31 51 712 C1 for "Titanium Based Hydrogen Storage Alloy With Iron and/or Aluminum Replacing Vanadium and Optionally Nickel," based on German Application DE 31 51 712 filed Dec. 29, 1981 by Otto Bernauer and Klaus Ziegler, and assigned to Daimler Benz AG. The key teaching of Bernauer, et al. are that the vanadium in a six component Ti-Zr-Mn-Cr-V-Ni alloy can be partially replaced by Fe and/or Al to give a lower cost thermal hydrogen storage alloy; that Ni can be partially replaced by Fe to further reduce the cost of the alloy; and that Fe can be used in the alloy without hurting its properties.
Specifically, Bernauer, et al. describe a thermal hydrogen storage alloy having the composition Ti.sub.1-a Zr.sub.a Mn.sub.2-x Cr.sub.x-y (V.sub.z Ni.sub.1-z).sub.y, where a is from 0 to 0.33, x is from 0.2 to 1.0, y is between 0.2 and x, and z is from 0.3 to 0.9. This patent discloses that the Ni is partially replaceable by Co and/or Cu, and from 1 to 5 atomic percent of the Ti is replaceable by strong oxygen getters, such as lanthanum and other rare earths. It is further disclosed that up to 20 atomic percent of the vanadium is replaceable by Al, with the provision that no more than 30 atomic percent of the vanadium can be replaced by Fe and Al; and that Ni atoms can be replaced by Fe atoms.
Multicomponent AB.sub.2 thermal hydrogen storage alloys of this general are also taught in German Patentschrift DE 30 23 770 C2 for "Titanium Manganese Vanadium Based Laves Phase Material with Hexagonal Structure, Used as Hydrogen Storage Material," based on German Application DE 30 23 770 filed Jun. 25, 1980 and DE 30 31 471 filed Aug. 21, 1980 by Otto Bernauer and Klaus Ziegler, and assigned to Daimler Benz AG. The key teaching of this patent is that the nickel in a six component Ti-Zr-Mn-Cr-V-Ni alloy can be partially replaced by Co and/or Cu to give a lower cost hydrogen storage alloy.
More specifically, the alloys disclosed in DE 30 23 770 have the formula Ti.sub.1-a Zr.sub.a Mn.sub.2-x Cr.sub.x-y (V.sub.z M.sub.1-z).sub.y in which M is one or more of the following: Ni, Co, and Cu; a is from 0.0 to 0.3; x is from 0.2 to 1.0; y is between 0.2 and the value of x; and the ratio of V to total Ni, Co, and Cu is between 9:1 and 3:2.
Matsushita Electric Industrial Company's U.S. Pat. Nos. 4,153,484 and 4,228,145, to Gamo, Moriwaki, Yamashita, and Fukuda, both entitled "Hydrogen Storage Material," disclose a class of C.sub.14 type Laves phase materials for the thermal storage of hydrogen. These materials are hydrided by gaseous hydrogen and dehydrided by evolving gaseous hydrogen. The disclosed C.sub.14 materials have a hexagonal crystal structure with an a lattice dimension of 4.80 to 5.10 .ANG. and a c lattice dimension of 7.88 to 8.28 .ANG.. The thermal hydrogen storage alloys disclosed in these patents contain Ti-Zr-Mn optionally with Mo and/or Cu. These patents require the presence of Mn; are silent as to V, Cr, or Ni; and contain no teaching of additional materials.
Other Laves phase materials are disclosed in Matsushita's U.S. Pat. No. 4,160,014 to Takaharu Gamo, Yoshio Moriwaki, Toshio Yamashita, and Masataro Fukuda for "Hydrogen Storage Material" claiming priority from Japanese Patent Application JP 52-054140 filed May 10, 1977. This patent discloses an AB.sub.a thermal hydrogen storage material where A is at least 50 atomic percent Ti and the balance is Zr and/or Hf; B is at least 30 atomic percent Mn with the balance one or more of the following; Cr, V, Nb, Ta, Mo, Fe, Co, Ni, Cu, and rare earths; and a is from 1.0 to 3.0.
Another class of AB.sub.2 thermal hydrogen storage materials is disclosed in U.S. Pat. No. 4,163,666 to D. Shaltiel, D. Davidov, and I. Jacob for "Hydrogen Charged Alloys of Zr (A.sub.1-x B.sub.x).sub.2 where A is one or more of the following: V, Mn, or Cr; and B is Fe and/or Co. The patent discloses this material as a hydrogen storage alloy.
Other prior art Laves phase-type hydrogen storage alloys are shown, for example in Matsushita Electric Industrial Co., Ltd.'s U.S. Pat. No. 4,195,989 to Takaharu Gamo, Yoshio Moriwaki, Toshio Yamashita, and Masataro Fukuda for "Hydrogen Storage Material" claiming benefit of Japanese Patent Application JP 53-044677 filed Apr. 14, 1978; and JP 52-130040 filed Oct. 28, 1977. This patent discloses a Laves phase hexagonal Ti-Mn-M alloy where M is one or more of the following: V, Cr, Fe, Co, Ni, Cu, and Mo; where the a lattice dimension is between 4.86 and 4.90 .ANG.; and the c lattice dimension is between 7.95 and 8.02 .ANG.. These materials are disclosed as thermal hydrogen storage alloys.
U.S. Pat. No. 4,397,834 to M. Mendelsohn and D. Gruen for "Method of Gettering Hydrogen under Conditions of Low Pressure" describes a ternary Zr-V-Cr hydrogen storage alloy. This alloy, having the formula Zr(V.sub.1-x Cr.sub.x).sub.2, where x is from 0.01 to 0.90, is used to getter or scavenge hydrogen gas.
In U.S. Pat. No. 4,406,874 to William E. Wallace, F. Pourarian, and V. K. Sinha, for "ZrMn.sub.2 -Type Alloy Partially Substituted with Cerium/Praseodymium/Neodymium and Characterized by AB.sub.2 Stoichiometry" discloses a thermochemical hydrogen storage alloy having the formula Zr.sub.x-1 M.sub.x Mn.sub.2 where x is between 0.0 and 0.3, and M is Ce, Pr, or Nd. The disclosed material is described as having a hexagonal Laves structure, an a lattice dimension of 5.00 to 5.03 .ANG., and a c lattice dimension of 8.20 to 8.26 .ANG.. This alloy is disclosed to be a thermochemical hydrogen storage alloy.
All of the AB.sub.2 hydrogen storage alloys described above are thermal hydrogen storage alloys.
Prior art Laves phase electrochemical hydrogen storage alloys are shown, for example, in Matsushita Electric Industrial Co., Ltd.'s Laid Open European Patent Application 0 293 660 based on European Patent application 88 10 7839.8 filed May 16, 1988 and claiming priority from Japanese Patent Applications JP 1-19411, JP 1-90698, JP 2-05683, JP 2-18698, and JP 2-58889; and the following Japanese Patents assigned to Matsushita:
1. JP 1-02855, for "Hydrogen Storage Alloy Electrode," issued Apr. 20, 1989 to Moriwaki, Gamo, and Iwaki, and was filed as Japanese Patent Application JP 2-58889 on Oct. 14, 1987. This patent discloses multi-dimensional hydrogen storage alloys and their hydrides. The alloys are disclosed to be C.sub.15 Laves phase type materials. These materials have the general chemical formula A.sub.x B.sub.y Ni.sub.z where A is Zr alone, or Zr and Ti and/or Hf, the Ti or Hf being 30 atomic percent or less; x=1.0; B is at least one of the following elements: Nb, Cr, Mo, Mn, Fe, Co, Cu, Al, and rare earth elements such as La and Ce; y=0.5 to 1.0; z=1.0 to 1.5; and the sum of y+z=1.5 to 2.5. This patent discloses that compositions of this general formula enhance the hydrogen storing ability of the alloy and suppress the loss of discharge capacity which occurs after repeated charge/discharge cycling (cycle life) of Ti-Ni and Zr-Ni binary systems. This patent contains no teaching of how to choose between Nb, Cr, Mo, Mn, Fe, Co, Cu, Al, La, and Ce substituent elements; or the relative proportions within this class of substituent elements which might yield optimal properties.
2. JP 63-284758, for "Hydrogen Storing Electrode" to Gamo, Moriwaki, and Iwaki issued Nov. 22, 1988, based on Japanese Patent Application JP 62-119411 was filed on May 15, 1987. This patent discloses an alloy which is expressed by the formula AB.sub.2, belongs to the Laves phase of intermetallic compounds, and has a cubically symmetric C.sub.15 structure and a crystal lattice constant in the range of 6.92 to 7.70 .ANG., where A represents Ti and/or Zr; and B represents V and/or Cr. This patent is silent as to additional substituents or modifiers.
3. JP 89/035863 for "Hydrogen Absorbing Electrode" to Gamo, Moriwaki, and Iwaki issued on Jan. 6, 1989 based on Japanese Patent Application JP 62-190698 filed on Jul. 30, 1987. This patent discloses an alloy of Zr, V, Ni that satisfies the general formula ZrV.sub.a Ni.sub.b, where a=0.01 to 1.20 and b=1.0 to 2.5. There is no specific teaching regarding substituents or modifiers.
4. JP 89/048370 for "Hydrogen Absorbing Electrode" to Gamo, Moriwaki, and Iwaki issued on Feb. 22, 1989 based on Japanese Patent Application JP 62-0205683 filed on Aug. 19, 1987. This patent discloses a alloy composition of the general formula ZrMo.sub.a Ni.sub.b, where a=0.1 to 1.2 and b=1.1 to 2.5. This reference contains no teaching or suggestion of complex alloys of five or more components.
5. JP 89/060961 for "Hydrogen Absorbing Electrode" to Gamo, Moriwaki, and Iwaki issued on Mar. 8, 1989 based on Japanese Patent Application JP 62-216898 filed on Aug. 31, 1987. This patent discloses an alloy composition of the general formula Zr.sub.a V.sub.b Ni.sub.c M.sub.d where a, b, c, and d are the respective atomic ratios of Zr, V, Ni, and M; a=0.5 to 1.5; b=0.01 to 1.2; c=0.4 to 2.5; d=0.01 to 1.8; b+c+d=1.2 to 3.7; and M is one or more elements selected from the group consisting of Mg, Ca, Y, Hf, Nb, Ta, Cr, Mo, Ti, W, Mn, Fe, Co, Pb, Cu, Ag, Au, Zn, Cd, Al, In, Sn, Bi, La, Ce, Mm, Pr, Nd, and Th. This patent, while it lists 28 metals plus mischmetal, does not teach or suggest any relationship between these metals.
Laid Open European Patent Application 02 93 660 describes hexagonal C.sub.14 Laves phase materials having a lattice dimension a from 4.8 to 5.2 .ANG.; and a lattice dimension c of from 7.9 to 8.3 .ANG.. The materials have the formula AB.sub.a where A is selected from the group of elements consisting of Zr, Ti, Hf, Ta, Y, Ca, Mg, La, Ce, Pr, Mm, Nb, Nd, Mo, Al, and Si; and B is selected from the group of elements consisting of Ni, V, Cr, Mn, Fe, Co, Cu, Zn, Al, Si, Nb, Mo, W, Mg, Ca, Y, Ta, Pd, Ag, Au, Cd, In, Sn, Bi, La, Ce, and Mm; where A and B are different from each other and a is from 1.0 to 2.5.
The only guidance provided by Laid Open European Application No. 02 93 660 in the selection of A components is that A is Zr, or a mixture of at least 30 atomic percent Zr, and the balance is one or more of the following: Ti, Hb, Si, and Al. The only guidance with respect to B is that B is V-Ni, Mo-Ni, or V-Ni-M in which M is another metal. In this Application, the subclasses of Zr-V-Ni, Zr-Mo-Ni, Mo-Ni, and Zr-V-Ni-M (where M is Mg, Ca, Y, Hf, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Pd, Cu, Ag, Xn, Cd, Al, Si, In, Sn, Bi, La, Ce, Mm, Pr, Nd, Th, or Sm) are particularly described. It is significant that Ti containing materials are excluded from this subclass, and that this application is silent as to any relationships and/or rules regarding the selection of the modifier or modifiers.
Another subclass disclosed in Laid Open European Application No. 02 93 660 is A'B'Ni, where Ai is Zr or at least 30 atomic percent Zr with one or more of the following elements: Ti, Hf, Al, and Si; and B' represents two or more elements chosen from the group consisting of Cr, Mn, Fe, and Co. This Application fails to disclose a modified, five or more component material based upon Ti-V-Zr-Ni-Cr, with additional metallic components to increase cycle life, cell voltage, capacity, discharge rate capability, low temperature performance, or any other desirable operational parameters.
C. Ti-V-Zr-Ni Type Materials
Another suitable class of electrochemical hydrogen storage alloys is the Ti-V-Zr-Ni type active materials used as the material of the negative electrode. These materials are disclosed in U.S. Pat. No. 4,551,400 (hereinafter the '400 Patent) to Krishna Sapru, Kuochih Hong, Michael A. Fetcenko, and Srinivasen Venkatesan, the contents of which are incorporated herein by reference. These materials reversibly form hydrides in order to store hydrogen. The materials used in the '400 Patent all utilize a generic Ti-V-Zr-Ni composition, where at least Ti, V, and Ni are present with at least one or more of Cr, Zr, and Al. The materials of the '400 Patent are multiphase materials, which may contain one or more AB.sub.2 phases with C.sub.14 and C.sub.15 type structures. One composition specifically disclosed in the '400 Patent is EQU (TiV.sub.2-x Ni.sub.x).sub.1-y M.sub.y
where x is between 0.2 and 1.0; y is between 0.0 and 0.2; and M=Al or Zr.
Two other illustrative compositions in the '400 Patent illustrate the partial substitution of Ti by Zr and/or Cr; EQU Ti.sub.2-x Zr.sub.x V.sub.4-y Ni.sub.y
where Zr is partially substituted for Ti; x is between 0.0 and 1.5; and y is between 0.6 and 3.5; and EQU Ti.sub.1-x Cr.sub.x V.sub.2-y Ni.sub.y
where Cr is partially substituted for Ti; x is between 0.0 and 0.75; and y is between 0.2 and 1.0.
It is, of course, understood from the '400 Patent, that both Zr and Cr may be partially substituted for Ti. Generally, the ratio EQU (Ti+Zr+Cr)/(V+Ni)
is from about 0.40 to about 0.67 to retain the proper Ni morphology in the hydrogen storage alloy.
The '400 Patent, however, is silent regarding the effects of additives and modifiers beyond those discussed above and as to the interactions between these additives and modifiers.
Other Ti-V-Zr-Ni materials may also be used for a rechargeable hydrogen storage negative electrode. One such family of materials are those described in U.S. Pat. No. 4,728,586 (hereinafter the '586 Patent) to Srini Venkatesan, Benjamin Reichman, and Michael A. Fetcenko for "Enhanced Charge Retention Electrochemical Hydrogen Storage Alloys and an Enhanced Charge Retention Electrochemical Cell," the disclosure of which is incorporated herein by reference. The '586 Patent describes a specific sub-class of the Ti-V-Ni-Zr hydrogen storage alloys comprising Ti, V, Zr, Ni, and a fifth component, Cr.
In a particularly preferred exemplification of the '586 Patent, the hydrogen storage alloy has the composition EQU (Ti.sub.0.33-x Zr.sub.x V.sub.0.67-y Ni.sub.y).sub.1-z Cr.sub.z
where x is from 0.00 to 0.25, y is from 0.1 to 0.6, and z is an amount effective for electrochemical charge retention, generally greater than 0.05 and less that 0.20; preferably about 0.07. These alloys may be viewed stoichiometrically as comprising 80 atomic percent of an V-Ti-Zr-Ni moiety and up to 20 atomic percent Cr, where the ratio of (Ti+Zr+Cr+optional modifiers) to (Ni+V+optional modifiers) is between 0.40 to 0.67. The '586 patent, while mentioning the possibility of additives and modifiers beyond the Ti, V, Zr, Ni, and Cr components of the alloys, are silent as to specific additives and modifiers, the amounts and interactions of the modifiers, and the particular benefits that could be expected from the modifiers.
A strong motivation for using the above described V-Ti-Zr-Ni family of electrochemical hydrogen storage alloys, as described in the '586 Patent is the inherently higher discharge rate capability of the materials. Important physical properties in this regard are the substantially higher surface areas for the V-Ti-Zr-Ni materials, and the metal/electrolyte interface. Measured in surface roughness factor (total surface area divided by geometric surface area), the V-Ti-Zr-Ni materials can have roughness factors of about 10,000. The very high surface area plays an important role in the inherently high rate capability of these materials.
The metal/electrolyte interface also has a characteristic surface roughness. The characteristic surface roughness for a given negative electrode electrochemical hydrogen storage material is important because of the interaction of the physical and chemical properties of the host metals, as well as of the alloys and crystallographic phases of the alloys, in an alkaline environment. The microscopic chemical, physical, and crystallographic parameters of the individual phases within the hydrogen storage alloy material are believed to be important in determining the macroscopic electrochemical characteristics of the hydrogen storage material. Since all of the elements, as well as many alloys and phases thereof, are present throughout the metal, they are also represented at the surfaces and at cracks which form the metal/electrolyte interface.
In addition to the physical nature of the roughened surface, it has been observed that the V-Ti-Zr-Ni materials tend to reach a steady state surface condition and particle size. This steady state surface condition is characterized by a relatively high concentration of metallic nickel. These observations are consistent with a relatively high rate of removal of the oxides of titanium and zirconium from the surface and a much lower rate of nickel solubilization. The resultant surface seems to have a higher concentration of nickel than would be expected from the bulk composition of the negative hydrogen storage electrode. Nickel in the metallic state is electrically conductive and catalytic, imparting these properties to the surface. As a result, the surface of the negative hydrogen storage electrode is more catalytic and conductive than if the surface contained a higher concentration of insulating oxides.
The surface, having a conductive and catalytic component, e.g., the metallic nickel, appears to interact with chromium alloys, in catalyzing various hydride and dehydride reaction steps. To a large extent, many electrode processes, including competing electrode processes, are controlled by the presence of chromium in the hydrogen storage alloy material, as disclosed in the '586 Patent.
Another reference that discussed the Ti-V-Zr-Ni class of materials is U.S. Pat. No. 4,849,205 to Kuochih Hong (hereinafter Hong) for "Hydrogen Storage Hydride Electrode Materials." Hong discloses four separate types of materials, each having four or five main components.
Hong's first material has the general formula EQU Ti.sub.a Zr.sub.b Ni.sub.c Cr.sub.d M.sub.x
(hereinafter formula 1) where 0.1&lt;a.ltoreq.1.4; 0.1&lt;b.ltoreq.1.3; 0.25&lt;c.ltoreq.1.95; 0.1&lt;d .ltoreq.1.4; 0.0&lt;x.ltoreq.0.20; a+b+c+d=3; and M=Al, Si, V, Mn, Fe, Co, Cu, Nb, of Ln's. Hong describes exemplary materials of formula 1 that have four components: Ti, Zr, Ni, and Cr, where up to 17 percent of the material is Cr. There is only one five component exemplary material of formula 1 described in Hong. This material uses Mn at a concentration of approximately 3.2 percent; no other exemplary formulas using a modifier with the basic four component system of formula 1 are disclosed. The only documented benefit of the exemplary alloys of formula 1 is enhanced charge capacity. Other benefits of the formula 1 material are suggested, i.e. long cycle life, but there is no data presented to support this claim or any other improved operational parameter. Table 1 of Hong shows that the inclusion of Mn with the four component material of formula 1 reduces the charge capacity compared to the other exemplary materials of formula 1. Thus, Hong teaches away from the use of Mn in a metal hydride battery system.
The second class of materials taught by Hong is expressed by the general formula EQU Ti.sub.a Cr.sub.b Zr.sub.c Ni.sub.d V.sub.3-a-b-c-d M.sub.x
(hereinafter formula 2) where 0.1&lt;a.ltoreq.1.4; 0.1&lt;b.ltoreq.1.2; 0.1&lt;c.ltoreq.1.3; 0.2&lt;d.ltoreq.1.95; 0.4&lt;x.ltoreq.0.20; 0.4&lt;a+b+c+d.ltoreq.2.9; and M=Al, Si, Mn, Fe, Co, Cu, Nb, or Ln's. Most of Hong's exemplary formula 2 compounds have only five components: Ti, Zr, Ni, Cr, and V. There is only one six component exemplary material of formula 2 described. This material uses Cu as a modifier at a concentration of approximately 3.2 percent; no other exemplary compounds that use a modifier with the basic five component system of formula 2 are disclosed. The only documented benefit of the exemplary alloys of formula 2 is enhanced charge capacity. Other benefits of the formula 2 material are suggested, i.e. long cycle life and good rate capability, but there is no data presented to support this claim or show an improvement in any other operational parameter. Table 1 of Hong shows that the inclusion of Cu as a modifier with the basic five component material of formula 2 reduces the charge capacity compared to the other five component materials of formula 2. Thus, Hong teaches away from the use of Cu in a metal hydride battery system.
The third class of materials taught by Hong is expressed by the general formula EQU Ti.sub.a Zr.sub.b Ni.sub.c V.sub.3-a-b-c- M.sub.x
(hereinafter formula 3) where 0.1&lt;a.ltoreq.1.3; 0.1&lt;b.ltoreq.1.3; 0.25&lt;c.ltoreq.1.95; 0.6&lt;a+b+c.ltoreq.2.9; 0.0&lt;x.ltoreq.0.2; if x=0, a+b.noteq.1.0, and 0.24&lt;b.ltoreq.1.3; and M=Al, Si, Cr, Mn, Fe, Co, Cu, Nb, or Ln's. Most of Hong's exemplary formula 3 compounds have only four components: Ti, Zr, Ni, and V. There is only one five component exemplary material of formula 3 described in Hong. This material uses Cu as a modifier at a concentration of approximately 6.2 percent; no other exemplary compounds that use a modifier with the basic four component system of formula 3 are disclosed. The only data presented for improved performance for exemplary alloys of formula 3 is for enhanced charge capacity. However, Table 1 of Hong shows that the inclusion of Cu in the four component material of formula 3 reduces the charge capacity compared to the other four component materials of formula 3. Thus, Hong teaches away from the use of Cu in a metal hydride battery system.
Finally, the fourth class of materials taught by Hong is expressed by the general formula EQU Ti.sub.a Mn.sub.b V.sub.c Ni.sub.d M.sub.x
(hereinafter formula 4) where 0.1&lt;a.ltoreq.1.6; 0.1&lt;b.ltoreq.1.6; 0.1&lt;c.ltoreq.1.7; 0.2&lt;d.ltoreq.2.0; a+b+c=3; 0.0&lt;x.ltoreq.0.2; and M=Al, Si, Cr, Mn, Fe, Co, Cu, Nb, or Ln's. Most of Hong's exemplary formula 4 compounds have only four components: Ti, Mn, Ni, and V. There is only one five component exemplary material of formula 4 described in Hong. This material uses Co as a modifier at a concentration of approximately 3.2 percent; no other exemplary compounds that use a modifier with the basic four component system of formula 4 are disclosed. The only data presented for improved performance for exemplary alloys of formula 4 is for enhanced charge capacity. However, Table 1 of Hong shows that the inclusion of Co in the four component material of formula 4 reduces the charge capacity compared to the other four component materials of formula 4. Thus, Hong teaches away from the use of Co in a metal hydride battery system.
It is important to note that while Hong discloses a rather lengthy "laundry list" of possible modifier materials, only two can truly be considered modifiers: Cu and Co, since the addition of Mn is disclosed in formula 4. Yet, no data is presented that there is any benefit from the use of Cu or Co. In fact, Hong teaches away from these modifiers since he only demonstrates capacity improvement, and the use of Cu and Co substantially reduces capacity. In addition, Hong is silent as to the intended functions of any components. Since the remaining modifier materials disclosed by Hong are neither employed in exemplary compounds, nor are discussed in light of their possible benefits the teaching value of Hong's "laundry list" is minimal at best. This is because one of ordinary skill could not determine from Hong any possible advantages to be expected from using modifiers singly or together.
D. AB.sub.5 Type of Hydrogen Storage Alloys
An alternative class of hydrogen storage alloys is the AB.sub.5 hydrogen storage alloys. These alloys differ in chemistry, microstructure, and electrochemistry from the AB.sub.2 and V-Ti-Zr-Ni-Cr types of electrochemical hydrogen storage alloys. Rechargeable batteries utilizing AB.sub.5 type negative electrodes are described, for example, in (i) U.S. Pat. No. 3,874,928 to Will for "Hermetically Sealed Secondary Battery with Lanthanum Nickel Electrode;" (ii) U.S. Pat. No. 4,214,043 to Van Deuketom for "Rechargeable Electrochemical Cell;" (iii) U.S. Pat. No. 4,107,395 to Van Ommering, et al. for "Overchargeable Sealed Metal Oxide/Lanthanum Nickel Hydride Battery;" (iv) U.S. Pat. No. 4,107,405 to Annick Percheron ne Guegon, et al. for "Electrode Materials Based on Lanthanum and Nickel and Electrochemical Uses of Such Materials;" (v) U.S. Pat. No. 4,112,199 to James D. Dunlop, et al. for "Lanthanum Nickel Hydride-Hydrogen/Metal Oxide Cell;" (vi) U.S. Pat. No. 4,125,688 to Bonaterre for "Negative Electrodes for Electric Cells" which discloses Hg modified LaNi.sub.5 negative electrodes; (vii) U.S. Pat. No. 4,214,043 to von Deuketom for "Rechargeable Electrochemical Cell," which discloses a LaNi.sub.5 -Ni cell; (viii) U.S. Pat. No. 4,216,274 to Bruning for "Battery with Hydrogen Absorbing Material of the Formula LaM.sub.5 " which describes a rechargeable cell with an AB.sub.5 type negative electrode of the formula LaM.sub.5 where M is Co or Ni; (ix) U.S. Pat. No. 4,487,817 to Willems, et al. for "Electrochemical Cell Comprising Stable Hydride Forming Material;" which discloses an AB.sub.5 type of material where A is chosen from mischmetal, Y, Ti, Hf, Zr, Ca, Th, La, and the rare earths, in which the total of Y, Ti, Hf, and Zr is less than 40 percent of the A component, and B is chosen from two or more members of the group of elements consisting of Ni, Cu, Co, Fe, and Mn, and at least one member of the group of elements consisting of Al, Cr, and Si; (x) U.S. Pat. No. 4,605,603 to Kanda, et al. for "Hermetically Sealed Metallic Oxide-Hydrogen Battery Using Hydrogen Storage Alloy," which discloses an AB.sub.5 electrochemical hydrogen storage alloy having the formula MNi.sub.5-(x+y) Mn.sub.x Al.sub.y, where M is chosen from the group consisting of lanthanum, lanthanides, and mischmetals, x and y are each between 0.0 and 1.0 and x+y is between 0.2 and 1.0; (xii) U.S. Pat. No. 4,696,873 to Yagasaki, et al. for "Rechargeable Electrochemical Cell with a Negative Electrode Comprising a Hydrogen Absorbing Alloy Including Rare Earth Component," which discloses AB.sub.5 alloys of the mischmetal-Ni-Mn-Al type; and (xiii) U.S. Pat. No. 4,699,856 to Heuts, et al. for "Electrochemical Cell," which discloses an AB.sub.5 material where A is chosen from mischmetal, Y, Ti, Hf, Zr, Ca, Th, La, and the rare earths, in which the total of Y, Ti, Hf, and Zr is less than 40 percent of the A component, B is chosen from two or more members of the group of Ni, Cu, Co, Fe, and Mn; at least one member of the group Al, Cr, and Si; including an activator chosen from the group consisting of Ni, Pd, Pt, Ir, and Rh.
It is clear from the above cited documents that the AB.sub.5 type alloys are a distinct and specific class of materials. Extensive work on processing techniques and electrode cell design demonstrate the singularity of AB.sub.5 technology, that is, that the AB.sub.5 technology represents a separate field of inventive effort from the AB.sub.2 and V-Ti-Zr-Ni-Cr classes of alloys. In particular, modification of AB.sub.5 type alloys must be viewed as practical only within the specific AB.sub.5 structure. This is due to the unique metallurgical, electrochemical, and oxidation characteristics of the AB.sub.5 class of alloys, especially regarding the use of lanthanum and other rare earths for electrochemical applications. Further, there is no prior teaching or suggestion regarding the selection and role of modifiers generally for the AB.sub.5 alloys or regarding specific performances that might result from specific modifiers.
E. Deficiencies of the Prior Art
While prior art hydrogen storage alloys frequently utilize various individual modifiers and combinations of modifiers to enhance properties, there is no clear teaching of the role of any individual modifier , or of the interaction or any modifier with other components of the alloy, or of the effects of any modifiers on specific operational parameters.
For electrochemical applications, which are substantially different from thermal hydrogen storage application, one must consider all performance attributes, such as cycle life, rate of discharge, discharge voltage, polarization, self discharge, low temperature capacity, and low temperature voltage.
While it is desirable to have alloys with all of these characteristics, it may also be advantageous to emphasize specific properties for a given application.
The prior art also fails to specify the role of particular modifications as well as how they work. Frequently, with AB.sub.2 and AB.sub.5 materials, there is a modifier, X, where X represents the rest of the Periodic Table. Such references teach away from the specific roles and functions of materials, and provide no practical benefit.
Further, the prior art does not consider the problem of excessive cell pressure that results from the use of hydrogen storage alloys in electrochemical cells; and hence, contains no teaching as to how these alloys might be modified to mitigate the problem.
As discussed above, charging and discharging hydrogen storage electrochemical cells involves the hydriding and dehydriding of metallic alloys concomitant with the electrolysis and reformation of water. These reactions involve the transfer of hydrogen atoms and during operation of the cells, particularly under conditions of high rate charge and discharge, significant hydrogen pressures can develop. Factors affecting hydrogen pressure in cell operation include the surface area of the cell electrodes, particularly the negative electrode; the formation of oxide layers on the electrodes; the catalytic activity of the oxides; and the equilibrium hydrogen pressure of the hydrided material. While the cells typically operate at pressures greater than atmospheric pressure, excessive hydrogen pressure is undesirable since it can result in a loss of aqueous-based electrolyte material, thereby limiting cell life. Also, if excess hydrogen pressure is not vented, the cell can burst, deform, or otherwise be destroyed.
Clearly, it is desirable to limit excessive hydrogen overpressure in electrochemical hydrogen storage cells; however, it is also equally important to maintain, or even improve, the other performance characteristics of the cells such as storage capacity, cycle life, self-discharge, and discharge rate. As described in detail below, the present invention is directed to improved alloys for use in hydrogen storage applications and particularly to alloys for use in hydrogen storage electrochemical cells, that have a low hydrogen overpressure during their operation. The alloys of the present invention also provide superior cell performance characteristics. These and other advantages of the present invention are readily apparent from the drawings, discussion, and description below.